1. Field of the Invention
This invention relates to inner covers for annealing or thermal processing furnaces.
2. Description of the Prior Art
Bell-type furnaces for annealing or other thermal processing of metal sheet, rod or wire are well known and typically comprise a base structure upon which the metal , generally in coil form, to be treated is supported either on end or by a carrier, a cylindrical-like inner cover closed at its upper end placed over the load its lower end sealed against the escape of an inert gas or other protecting atmosphere circulated therein about the load by a blower or fan incorporated in the base structure, and an insulated generally cylindrical or rectangular furnace bell closed at its top enclosing the inner cover. The furnace bell includes means for heating the enclosed inner cover and load, as desired. The inert or protective atmosphere within the inner cover maintains a non-oxidizing carbon-controlling or other processing atmosphere within the inner cover during the processing of the load.
The lower open end of the inner cover is typically sealed by being imbedded in sand or suitable liquid. Other types of seals utilizing rubber or refractory fiber are also known.
Because of their use inner covers are repeatedly heated and cooled during use. They deteriorate rapidly principally because of the non-uniform expansion and contraction, etc., to which they are subjected during repeated use and thus have a relatively short, useful life. The most rapid deterioration takes place around the lower open ends of the covers and is extremely bad in covers used with direct firing burners.
Covers should be able to withstand the rigors of thermal cycling and mechanical handling but at the same time they should be as light in weight as possible both for efficiency in energy usage (heat transfer) and convenience in handling.
A typical furnace and interior cover of the character to which this invention relates is disclosed in U.S. Pat. No. 2,815,197.